sanitize region input

[pseudonym117]

region is sometimes user-controlled and allows for redirect attacks
in order to steal API keys. This is no longer possible.

[codecov]

Codecov Report

Merging #197 (21faff6) into master (368cd2c) will decrease coverage by 0.08%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master     #197      +/-   ##
==========================================
- Coverage   93.81%   93.72%   -0.09%     
==========================================
  Files          83       85       +2     
  Lines        1002     1020      +18     
  Branches       69       71       +2     
==========================================
+ Hits          940      956      +16     
- Misses         41       42       +1     
- Partials       21       22       +1     

Impacted Files	Coverage Δ
src/riotwatcher/LolWatcher.py	94.73% <ø> (ø)
src/riotwatcher/LorWatcher.py	100.00% <ø> (ø)
src/riotwatcher/TftWatcher.py	100.00% <ø> (ø)
src/riotwatcher/ValWatcher.py	100.00% <ø> (ø)
src/riotwatcher/riotwatcher.py	100.00% <ø> (ø)
src/riotwatcher/Handlers/IllegalArgumentError.py	66.66% <66.66%> (ø)
src/riotwatcher/Handlers/SanitationHandler.py	100.00% <100.00%> (ø)
src/riotwatcher/Handlers/__init__.py	100.00% <100.00%> (ø)
src/riotwatcher/__init__.py	100.00% <100.00%> (ø)
src/riotwatcher/exceptions.py	100.00% <100.00%> (ø)

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